1. Field of the Invention
This invention relates to the field of control circuits for video signals in television receivers and video monitors.
2. Description of Related Art
Excursions of signals related to the luminance of a video signal beyond a white level threshold can cause problems in a television receiver or in a video monitor. Excessive beam currents in the cathode ray tube of a television receiver, for example, can result in a degraded image. Such excess beam currents can adversely affect the performance of the receiver deflection system for the cathode ray tube, can cause electron beam spot defocusing and can cause picture blooming. High beam currents which exceed the safe operating capacity of the cathode ray tube can result in damage to the cathode ray tube itself, as well as to other circuits or components in the receiver.
Many circuits are known for controlling average beam currents, by monitoring a signal representative of the magnitude of the cathode ray tube beam current derived from the cathode ray tube high voltage resupply system of the receiver. In one such arrangement, as described in U.S. Pat. No. 4,167,025--Willis, a beam current limiter responds to the current resupply derived signal when the signal exceeds a threshold level.
A cathode ray tube for a color receiver has a tendency to shift color purity in any area where there is localized heating of the shadow mask, for example where there is a relatively large white area. This condition is often referred to as blistering. The condition is usually worse on larger tubes, for example those having larger screen sizes, such as 27 inch or 31 inch diagonal measurements. Monitoring average beam current is generally inadequate to detect and prevent localized peaks of beam current, which are those responsible for localized shadow mask heating. The accepted method for preventing blistering is to reduce the level of video drive so that beam current in the white area is not high enough to cause discoloration. However, larger television screens are often also darker. Limiting the beam current in this fashion may make the displayed picture unnecessarily dim, especially for small white areas.
In order to cope with the problems of peak beam currents, not reflected in values of average beam current, other beam limiter circuits have been developed. In U.S. Pat. No. 4,599,643-Harlan, excessive beam currents are limited by means which operate independently of the cathode ray tube current resupply system. Instead, a combined signal representative of the combined instantaneous magnitudes of plural color signals is utilized. Beam current is limited, for example by reducing contrast, if the combined signal exceeds a threshold. Another beam limiter operating responsive to a combined color video signal is disclosed in U.S. Pat. No. 4,689,668--Sutherland, II, et al. The control circuit includes, in the following order, a first low pass filter, a first peak detector, a second low pass filter, and a second peak detector. The second peak detector develops a control signal for limiting the amplitude of the video signal. The second low pass filter has a time constant of about 5 V, where V is the period of a vertical field. This is substantially greater than the time constant of the first peak detector, which is about 0.05 H, where H is the period of one horizontal scanning line. In practice, the circuit will respond to beam currents conducted by white areas as small as approximately a full screen height vertical bar pattern having a width of about 10% of the screen. Wider patterns of lower height will also result in control of beam current by the circuit. In either case, persistence of the condition must continue for successive vertical fields in order for the respective thresholds to be surpassed and beam current limited.
Although the foregoing circuits have proved effective for preventing blistering in the shadow masks of the cathode ray tubes in conjunction with which they were developed, these circuits have not proved entirely satisfactory for use with the larger darker screens now more commonly available. The foregoing circuits have also shown a certain lack of responsiveness to lower level peak beam current conditions. These circuits are intended for use with receivers which are used not only with television program material, but computer displays and video games as well. Computer displays and video games are more likely to generate the geometrical patterns responsible for more serious shadow mask blistering and warping.